The flu shot: 1.4 million Americans have already received this year's version, but when 22-year-old Stephanie Molony got hers, it gave her a nasty surprise.
"Extremely tired. I had a really bad headache, I was nauseous. A lot of back pain," she said.
Stephanie's among the 14-20 percent of the population expected to experience more than just a sore arm.
Infectious Disease Specialist Dr. Andy Miller says roughly one in five will run a low grade fever and suffer what doctors call malaise.
"Malaise means feeling a little crappy, feeling a little tired, having lower energy than usual," he said.
Howl long do symptoms last?
"Usually 24 to 48 hours but not much longer."
With the exception of extreme reactions, side effects tend to be under-reported as people complain to each other and not necessarily to their doctor. Still, a reaction one year doesn't mean one the next. Each year's vaccine is a little different from the previous year's.
Doctors point out that the unpleasant reactions should really be taken in a positive light. Dr. Miller says that means the flu shot is working.
"A day of malaise compared to 10 days out of work with severe flu, it's definitely worth it," he said.
Doctors advise acetaminophen to ease the discomfort.